1. Field of the Invention
The invention relates to navigation receivers and, more particularly, to very high frequency omni-range (VOR) receivers commonly used for aircraft navigation.
2. Description of the Prior Art
Currently, a primary source of in-flight aircraft navigation information is provided through the very high frequency omni-range (VOR) system. This system is typically comprised of a plurality of VOR transmitters (stations) which are widely dispersed throughout a given geographic territory. Each station within this territory continuously transmits a modulated signal on a unique pre-assigned carrier frequency with respect to all the other stations in the territory.
For navigation purposes, a VOR signal is comprised of two components: a 30 Hz reference sinusoidal signal and a 30 Hz variable sinusoidal signal. The reference signal is transmitted omni-directionally from the VOR station, such that its phase remains constant anywhere along the periphery of a circle concentrically drawn at any radial distance outwardly from the station. Hence, for any two aircraft flying different magnetic course headings toward the station, i.e., located at a different point along the periphery of the circle, each aircraft receives the same reference signal as that of the other, that is without any phase difference existing therebetween. The variable signal is transmitted such that the amount of phase difference between it and the reference signal linearly varies from 0.degree. to 360.degree. along the periphery of any concentric circle centered about the station. The pattern of the variable signal is such that the phase difference at any point along the circle is determined by the magnetic bearing of that point measured from the station. For example, if an aircraft were to fly a magnetic heading of 0.degree. from a VOR station, the reference and variable signals would be totally in phase; consequently, any phase difference existing therebetween would be 0.degree. . If, alternatively, an aircraft were to fly a 90.degree. magnetic heading from the station, the variable signal would then lag the reference signal by 90.degree. resulting in a 90.degree. phase difference existing therebetween. Likewise if an aircraft were to fly a magnetic heading of 216.degree. from the VOR station, then there would be a 216.degree. phase difference existing between the received variable and reference signals occurring at that point (and so on). It is this varying course dependent phase difference that provides the airborne navigation information.
A VOR receiver mounted in an aircraft and tuned to the carrier of a desired VOR station determines the difference then appearing between the reference and variable signals transmitted by that VOR station and, in response thereto, provides a visual indication of the magnetic course bearing then being flown with respect to that station. To minimize reception error in differentiating between the reference and variable signals, the VOR signal is transmitted with the reference signal frequency modulating a 9960 Hz subcarrier. This subcarrier and the variable signal each amplitude modulate the main VOR carrier by approximately 30%. In addition, aural identification of the VOR station is provided by a unique audible Morse Code signal that also modulates the main carrier.
To effectively indicate a present magnetic course, i.e., a particular VOR radial, a combined omni-bearing selector (OBS) and a left/right indicator were developed in the art for connection to the output of the VOR receiver. The OBS permits the pilot to select any magnetic course (from 0.degree. to 360.degree. ) with respect to a particular selected VOR station, by simply turning a knob on the indicator until the selected magnetic course, i.e., the associated VOR radial, is indicated. The VOR receiver determines the actual radial that is then being flown and, in response to information regarding the selected course, generates a deviation voltage. This voltage is zero-valued whenever the aircraft is flying on the selected course. Or the voltage is positive or negative depending upon whether the aircraft is on one side of the selected course or the other. The deviation voltage is applied to an analog meter (left/right indicator) which indicates the course deviation as a left or right movement of its needle. Deviation information is presented such that whenever the OBS selector is set to the correct (non-reciprocal) magnetic course, any course deviations can be eliminated, i.e., the pilot can fly towards the correct course, by flying "into the direction" of the movement of the needle. For example, a left or right off-course indication can be corrected by turning the aircraft to the left or the right, respectively.
Well known VOR indicators of this type also include "To/From" indicator which specifies whether the course set through the OBS selector is in a direction that will take the pilot towards ("To") or away from ("From") the station. The "To/From" indicator is generally positioned in very close proximity to both the OBS selector and course deviation needle such that the pilot can easily perceive all three simultaneously.
In VOR receivers currently known to the art, two tuned circuits, one tuned to 9960 Hz and the other tuned to 30 Hz, are used to separate the reference and variable sinusoidal signals from the main VOR carrier. Each separated signal is then applied, through appropriate limiters and detectors, to an appropriate input of a phase detector. The phase detector produces the deviation voltage which is then applied through an amplifier to drive the left/right and "To/From" indicators.
Unfortunately, the phase detector used in presently existing VOR receivers is highly susceptible to both amplitude and phase variations occurring between its two input (reference and variable) signals. These variations are often produced by factors unrelated to actual course deviations --such as for example reflected VOR signals, transient voltages occurring within the aircraft, weak VOR signals, or amplitude modulations imparted to the received VOR signal caused by movement of the aircraft's propeller or rotor --and thus produce erroneous course deviation indications on the left/right needle.
Specifically, reflections of VOR signals are often caused by the physical surroundings of the VOR station, and specifically by the presence of man-made objects such as buildings, power lines, bridges and/or terrain anomalies. Inasmuch as these effects are quite common and frequently occur near most VOR stations, reflected signals often reach an airborne VOR receiver at all phase angles and with an amplitude that varies as the aircraft flies along any selected radial. These reflections, in turn, cause an amplitude variation between the received reference and variable signals and also corrupt the correct phase relationship occurring therebetween.
It is well known that the physical environment around any VOR station causes the amount of received VOR signal attributable to reflections to vary from point to point along any radial emitted by that station. Moreover, the differences in the physical environments associated with any two VOR stations usually results in differing amounts of reflections for points along any particular radial of one station with respect to identically situated points associated with the other station. Therefore, the received VOR signal at most points along any radial from any VOR station usually contains a signal portion attributable to reflections. This signal portion usually produces some error in the course deviation indication produced by the VOR receiver.
One possible solution to minimizing reflection-based errors is to store a table of correction factors within the VOR receiver. Each correction factor would be accessed by both the radial selected by the OBS selector and the radial distance from the present position of the aircraft to the VOR station. The accessed correction factor would be used to offset the left/right indication to compensate for the expected reflection occurring at that position. Unfortunately, the number of separate correction factors needed for one VOR station can be substantial. Since such information in all likelihood would be required for a plurality of stations, memory requirements to accommodate such a table can be quite significant. Hence, the resultant circuitry of the VOR receiver would likely become disadvantageously complex, bulky and excessively expensive. Moreover, since the physical environments around VOR stations are apt to change from time to time, as a result of, for example, new structures or man-made modifications to nearby terrain, any such table would, of necessity, require updating which would, in turn, further disadvantageously escalate the cost of the VOR receiver.
An alternate solution, particularly with the advent of increasingly sophisticated micro-processors, might appear at first glance to lie in storing data relating to the physical environment of a particular VOR station and then calculating the necessary correction factor for each point on an aircraft's path along a radial emitted from that station and using the result to appropriately correct the left/right needle indication. However, the immense complexity associated with such a calculation (both in terms of the mathematics involved as well as the substantial processing time required) effectively eliminates this solution as a viable possibility.
Consequently, presently existing VOR receivers known to the art are generally devoid of any provision to eliminate commonplace and substantial course deviation errors attributable to reflected VOR signals. Therefore, since the inception of the VOR radio navigation system, the aviation community has come to rely on the Federal Aviation Administration (FAA) to judiciously select sites for VOR stations in which the physical environment of each is least likely to produce reflections. Unfortunately, every site produces some reflections and thus necessitates substantial expenditures of time --including flight checks, and money --in assessing its suitability for a VOR station. As an expedient in selecting sites, the FAA has developed standards for maximum amounts of course deviation error which under various categories of VOR use can be safely tolerated for any VOR station. Hence, every pilot learns to disadvantageously accept a certain amount of error in radio navigating with regard to any VOR radial.
A further deficiency of presently existing VOR receivers involves the so-called "cone of confusion." In particular, whenever an aircraft is very close to a VOR station, the "To/From" indicator fluctuates wildly and the left/right needle rapidly moves back and forth from one extreme point of its travel to the other. The region above a VOR station in which these indications occur generally resembles an inverted cone with the VOR station at its apex. With presently existing VOR receivers, the sides of the cone are inclined with respect to its vertical axis at angles of approximately 30.degree. . The width of the cone varies with altitude and normally extends for several miles at an altitude of several thousand feet above any VOR station. While these indications are used by the pilot as an "over-station" indication, these indications, due to the varying altitude dependent width of the cone, disadvantageously provide no accurate indication of the exact point in time when the aircraft is directly over the VOR station.